Re: Exclusive file access

[Simon Wright <simon@pushface.org>]

ahlan@marriott.org writes:

> Bruce B. tells me that there are routines in the package Directories
> that can be used to check if the file is already open. However that
> wouldn't be a full solution because there would be nothing to stop any
> other program from opening the file and messing with it. However it
> would be a start. If the file was already opened (by some other
> process) then I could avoid opening it. Unfortunately I couldn't find
> any such routine in Ada.Directories. Am I going blind?

GNAT.Directories perhaps? though I only see an Is_Open for directories,
not files.

GNAT.OS_Lib (a renaming of System.OS_Lib)? no, don't think so.

> So far the only solution I can see is for me to write my own filing
> system, one that has the ability to open the file for exclusive file
> access. However I cannot believe that this is what people really do.
> As it stands if I write a program that writes a log to a text file a
> second occurrence of the program if run concurrently will mess up this
> log.
> Is there really no way for an Gnat Ada program to open a file for
> exclusive access against other processes without resorting to the
> program writing his/her own filing system? (Open/Read/Write/Close that
> call directly the OS API)

I misunderstood you. To me, "filing system" means the whole organisation
of files with hierarchies of directories.

I think you are rather out of luck. I can see how it might be done on OS
X [open(2) with O_EXLOCK, then write(2)].

But, if you want a logger, what about for example Alog[1]? It's probably
overkill. But it's an exact analog of how you'd do it for tasking within
one process: open the file in a dedicated task, and send messages to be
logged to the task.

[1] http://www.codelabs.ch/alog/